Field of the Invention
The present invention concerns a method to generate image data (in particular magnetic resonance exposures) of a subject moving inside a body, for example an organ, parts of an organ or an arbitrary target structure inside a human or animal. Moreover, the invention concerns an image data generation device to generate image data of a subject moving inside a body, as well as a magnetic resonance system with such an image data generation device.
Description of the Prior Art
In order to obtain image data (magnetic resonance exposures) from a region of the inside of the body of an examination subject in magnetic resonance tomography, the body or the body part to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field (most often designated as a B0 field). Nuclear spins in the subject are thereby aligned parallel to the direction of the B0 field (typically designated as the z-direction). Moreover, radio-frequency pulses are radiated into the examination subject with radio-frequency antennas, the frequency of the radio-frequency pulses being at or approximately at the resonance frequency (known as the Larmor frequency) of the nuclei to be excited (normally hydrogen nuclei) in the basic magnetic field. These radio-frequency pulses are therefore designated as magnetic resonance radio-frequency pulses in the following. The magnetic flux density of these radio-frequency pulses is typically designated with B1. By means of these radio-frequency pulses, nuclear spins of the atoms in the examination subject are excited such that they are deflected by an amount known as an “excitation flip angle” (generally shortened to “flip angle”) out of their steady state (parallel to the basic magnetic field B0). The nuclear spins then initially precess around the z-direction and relax again bit by bit. The in-phase revolution of the microscopic spins around the precession cone can be viewed as a macroscopic nuclear magnetization in the x/y plane (orthogonal to the z-direction). The magnetic resonance signals generated in this relaxation of the nuclear magnetization are acquired as raw data by radio-frequency reception antennas. Magnetic resonance images of the examination subject are reconstructed on the basis of the acquired raw data. Spatial coding of the magnetic resonance signals takes place with the use of rapidly switched (activated) gradient magnetic fields that are superimposed on the basic magnetic field during the emission of the magnetic resonance radio-frequency pulses and/or the acquisition of the raw data. In the data acquisition, the raw data are initially associated with frequency domain (known as k-space). The reconstruction of the image data then takes place by means of a Fourier transformation of the raw data of k-space into the image data domain (spatial domain).
Particularly when image data of a complete organ should be generated (i.e. when either volume data or a number of densely adjoining slice images should be acquired that cover the complete region in which the organ is located) a longer measurement time is required. This is a problem if a moving organ is being examined (the heart, for example). The heart not only performs its own cyclical movements due to the heart beat, but also is additionally subject to movement due to the breathing of the patient. If raw data were simply acquired during the different movement cycles, and image data were then reconstructed from this raw data, this would lead to significant movement artifacts (for example smearing in the images), such that the generated images could often not be used for a reasonable diagnosis.
One possibility to avoid such artifacts is to acquire raw data only in a defined movement phase of the organ (heart), for example to trigger the raw data acquisition in a suitable manner. For example, with regard to the heart movement it is possible to implement such a triggering at a defined heart phase with the use of an EKG. The data are preferably acquired in the diastolic phase, since in this phase the heart movement is minimal for a relatively long time. This diastolic phase lasts approximately 100 ms. In many acquisition methods (in particular in the acquisition of the complete volume or a number of densely placed slices that cover the heart), a duration of 100 ms is not sufficient to acquire all raw data. This particularly applies in the acquisition of raw magnetic resonance data. Therefore, raw data sets or, respectively, segments of k-space must respectively be acquired in the matching cardiac phase in order to fill k-space with the necessary raw data before the image data of the volume or the desired slices are then reconstructed in the manner described above. However, the breathing movement that is additionally superimposed on the movement of the heart can lead to the situation that the heart can be situated at different locations in the body (for example in the diastolic phases of successive cardiac cycles) depending on the current movement phase of the breathing cycle. In principle, it would naturally be possible to also monitor the breathing cycle with appropriate devices and to additionally trigger on the breathing cycle. In such a case, however, raw data would only be acquired when the desired cardiac phase and the desired breathing phase randomly occur together. This would lead to a total measurement duration that is much too long, and therefore such measurement methods are not acceptable in practice.
For example, a method for time windowing of angiographic magnetic resonance acquisitions under consideration of breathing phases is described in the article “Respiratory Self-Gated Four-Dimensional Coronary MR Angiography: A Feasibility Study” by Peng Lai et al. in Magn. Reson. Med., Vol 59, 2008, No. 6, P. 1378-1385.
In order to fill a sufficiently large k-space volume with measurement data so as to be able to reconstruct from these a three-dimensional volume of a complete organ (such as the heart), it is consequently necessary to acquire raw data at different measurement points in time in different movement phases of the subject. This means that the scan patterns are deconstructed into different segments, and that the segments or raw data sets are most often read in directly successive cardiac cycles that can be located in different movement phases of the subject due to the breathing. A typical scan pattern is known as a phyllotactic, spiral-shaped pattern, as will be explained below in further detail. In this pattern, readout points lie on a spoke-like trajectory as seen in a plane (for example the x/y plane) through k-space, wherein the spokes proceed outwardly from the k-space center and are curved in a spiral shape, for example. The individual readout (entry of raw data into) of each plane of such spokes proceeds plane-by-plane in a straight line in the direction (for example the z-direction) orthogonal to the planes. For example, a complete filing of all planes among the z-direction through k-space is considered as a readout process.
However, the problem exists that the raw data originate from different movement phases (in particular breathing phases), independently of in which segments the raw data are acquired. A correction within the reconstruction method is therefore required, which compensates again for these image disruptions caused by the different movement phases, and thus ensures that no movement artifacts appear in the finished image data.
In the article “3D Radial Sampling and 3D Affine Transform-based Respiratory Motion Correction Technique for Free-breathing Whole-Heart Coronary MRA with 100% Imaging Efficiency” by Himanshu Bath et al. in Magn. Reson. Med., Vol. 65, 2011, No. 5, P. 1269-1277, it is proposed to generate respective image data from the raw data from different movement phases, and to then generate a movement-corrected image from these image data via suitable summation. In DE 197 13 846 A1 and DE 697 21 900 T2, corrected image data are similarly created from previously reconstructed image data.